Electroconductive transparencies, such as those used in electrophotographic processes, require high light transmission and rather low surface resistivity. They also must be mechanically strong and resistant to abrasion and layer separation.
Transparent, electrically conductive film structures are known. British Pat. No. 1,417,628 discloses an electrophotographic film having an organic, polymeric transparent substrate and a layer of noble metal sputtered thereon. The noble metal is, in turn, coated with a layer of a photoconductive, polymeric material. According to this reference, adhesion between the metal layer and the substrate is strong. Apparently, no special treatment of the substrate's surface is necessary prior to the application of the metal, although this disclosure suggests that adhesion may be improved by coating the substrate with a thermosetting polymer. Metal sputtering, however, is not a desirable technique because it usually can be done only at slow rates to improve adhesion to the base film. Simple, continuous metal vaporization techniques also have been employed in the past, but the adhesion of the vaporized metal to the base film was deficient. Furthermore, the abrasion resistance of both vaporized and sputtered films was poor whether the base, organic polymer film was untreated or electrically or thermally precleaned. Various disclosures teach primer coatings to improve adhesion between a substrate film and a layer of material applied thereto. For example, British Pat. No. 1,264,338 discloses a number of suitable materials other than "heat-sealable coating polymers". This reference suggests that the primer-coated base film is more receptive to subsequent coating, printing, or "metallising". However, no further teaching as to such metallizing process or products is provided. It appears that this disclosure contemplates reflective coatings of substantial thickness, such as those used in printing and the like. Prior art electrophotoconductive, transparent, film laminates in which the metal layer thickness was so chosen that the laminates had high optical transparency and at the same time acceptable surface resistivity were deficient in interlayer adhesion and resistance to abrasion. Improvement of these two properties has long been sought.